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I decant my seedlings into 6 or 7 inch wide (top diameter) pots. Once they have a few leaves, I feed with approximately one tablespoon of chopped lucerne hay, topped with the same amount of sheep manure. Repeat about every four weeks. Later, when they are really on their way, a quarter teaspoon of sulphate of potash.

Hubert, they don't need much fertilizer initially, It's always best to err to the side of caution. I sometimes wait until they have true leaves to start.

I suggest quarter strength fish emulsion. Once they initiate true leaves increase to half strength and once they are hardened off completely and established full strength.

You can substitute original Miracle Gro at similar ratios, but I've found the fish fertilizer also helps prevent damping off early on.

It's pretty much impossible to overfeed roses. They have huge appetites. As long as you're following the recommended mixing rate and not actually drowning the roses, it's unlikely to hurt them. I'd also follow the recommended frequency of feeding.

Amount of water needed will depend on conditions and on the size of the bush. Ditto for amount of food. In hot (Australian) weather I might throw 20 litres of water at a medium size bush in one watering, but they'd only be watered every few days at most. This is assuming they are in the ground, not potted, and are established.

In winter I might not water them at all for a week or two, depending on conditions. Easy way of telling is to stick your finger into the topsoil just beneath your mulch. If the top couple of inches of soil is feeling dryish, water them well. If it's moist, don't bother.

It's a good idea to not just rely on packaged fertilisers and to also give roses a nourishing mulch. Manure is great. Lucerne is great. Lay it on thick if the weather is hot. Roses like cool roots.